1. Field of the Invention
This invention pertains to the connection of integrated circuit chips in an electrical circuit.
2. Description of Related Art
Conventional techniques for attaching integrated circuit chips to circuit traces on a printed circuit board or flexible conductor take up a considerable amount of space so that the chips cannot be positioned as close to each other as desired for many installations. A high density of chips may be essential in an increasing number of electrical circuits. In conventional chip attachment a chip is positioned on a substrate with its pads facing outwardly and wires are extended between the pads of the chip and the circuit traces or pads on the substrate. Obviously, space must be provided around the chip to accommodate the connecting wires.
In order to alleviate this problem what is known as the flip chip or controlled collapse chip connection has been devised. In this arrangement, solder bumps are plated onto the pads of the integrated circuit chip. The chip then is inverted with its pads positioned over and pressed against the circuit traces. Upon melting and fusion of the solder, connections can be achieved.
This successfully increases the permissible density of the chips on a substrate, but has serious drawbacks. Unless all of the solder bumps on the chip properly engage the corresponding circuit traces, a solder joint will not be achieved. When this occurs, it becomes necessary to scrap the part. The solder bumps are destroyed upon heating and cannot be reworked. For multichip modules it may then become necessary to discard a very expensive component which has had a number of prior operations performed on it before the attempted connection of the chip. Inasmuch as it is impossible to achieve complete assurance that all of the chips will bond properly in the normal flip chip operation, the expense may become prohibitive for multichip modules.
Also, it is necessary to apply the solder to the chip at wafer level of fabrication. Not all chip manufacturers are willing to supply chips with solder bumps, and some manufacturing methods preclude solder bumps at all. Thus only a limited number of chips are available for flip chip. To use flip chip methods, all the chips on a module should be attached via flip chip. If some of the chips are not available with solder bumps, then it may not be economical to use flip chip processing at all.
Similar problems are encountered when posts are formed on the chip pad or when bumps are provided with tape automated bonding.